These studies are expected to provide new information on the relationships between the metabolism and mechanical peformances of the heart under normal and pathological conditions. We will use a microl oxygen electrode to measure the PO2 in the heart in situ, in the isolated whole heart, and in a strip of isolated heart muscle. We will monitor as many other parameters as feasible and appropriate including arterial pressure, blood flow, and blood gases and chemical content, along with several indices of heart performance. A variety of agents and experimental conditions will be used, and several species tested. From these experiments we will, first of all, determine the normal values for PO2 and how the PO2 varies under a variety of conditions. The experiments are designed to reveal: a) new information on the energetics of heart muscle; and b) the factors which normally regulate coronary blood flow. Subsequently, we will perform similar studies on animals in varous types of shock and in experimental failure in an effort to elucidate the defects.